Wesnoth, SuperTuxKart, 0AD or OpenMW are widely known. Being a bit ill these days I went through the FSF game directory, searching for FLOSS games that I'm unaware of. Maybe this can become a collection-thread for more obscure, second line projects?
3dyfication of a old atari 2d game.
https://sourceforge.net/projects/adaventure/
Survival 2d game, active development:
https://cataclysmdda.org/
Neverball: Game of Skill, comparable to the Brio wood labyrinth game.
https://neverball.org/index.php
Pianobooster: Educational Guitar-Hero-Clone for Pianos, aiming to train real piano skills:
https://www.pianobooster.org/index.html
Battle for the solar system, narrative 2D-Space-shooter:
https://www.battleforthesolarsystem.com/games/pw/
Simple grand strategy game. Development seems death, but description is promising.
http://a-nikolaev.github.io/curseofwar/